1. Field Of The Invention
This invention relates to a hydraulic height adjusting device, more particularly to a height adjusting device which is simple in construction and which is easier to manufacture than the prior art.
2. Description Of The Related Art
Referring to FIG. 1, a conventional height adjusting device 1 includes an inner cylinder 11 and an outer cylinder 12. Stoppers, 13 and 14, plug both ends of the outer cylinder 12 and the inner cylinder 11 which is disposed inside the outer cylinder 12. A clearance 121 is left between the outer and inner cylinders, 11 and 12. A helical spring 17 is disposed in the clearance 121 around the inner cylinder 11. A piston 151 is provided inside a fluid reservoir 111 confined by the inner cylinder 11. A piston shaft 15 is connected to the piston 151 and extends through the stopper 14. A slidable piston 18 is disposed in the clearance 121 between the stopper 13 and the helical spring 17. The slidable piston 18 and the stopper 13 cooperatively confine a fluid receiving space 181. The stopper 13 has a longitudinal shaft opening 132 and a pair of fluid passages 131 which communicate the fluid receiving space 181 with the fluid reservoir 111 confined by the inner cylinder 11 via the shaft opening 132. A shaft 19 has a lowermost end extending through the shaft opening 132 and into the fluid reservoir 111. A valve piece 193 is provided on the lowermost end of the shaft 19 to selectively block the shaft opening 132. The uppermost end of the shaft 19 extends through the stopper 13. A switching member 191 is provided on the uppermost end of the shaft 19. A helical spring 192 is disposed between the switching member 191 and the stopper 13, and surrounds an uppermost portion of the shaft 19. The helical spring 192 is attached to the switching member 191 at one end and is attached to the stopper 13 at the other end so as to bias the valve piece 193 to tightly block the shaft opening 132 in order to prevent fluid from flowing between the fluid reservoir 111 and the fluid receiving space 181.
When incorporating the height adjusting device 1 in a height adjustable chair, the lowermost end of the piston shaft 15 is fixed to the chair support stand (not shown). The switching member 191 should be conveniently located and is operable by means of a lever or any similar device (not shown).
To adjust the height of a chair incorporating the height adjusting device 1, the switching member 191 is actuated to move downward, thereby compressing the helical spring 192 and moving the shaft 19 downward to correspondingly remove the valve piece 193 from the shaft opening 132. The topmost end of the stopper 13 supports a load-bearing article, such as a chair seat (not shown). When downward pressure is applied to the stopper 13, the outer and inner cylinders, 12 and 11, simultaneously move downward. Since the valve piece 193 is displaced from the shaft opening 132, hydraulic fluid inside the fluid reservoir 111 is forced by the piston 151 to flow through the shaft opening 132 and the fluid passages 131 and into fluid receiving space 181. As the volume of hydraulic fluid inside the fluid receiving space 181 gradually increases, the piston 18 consequently moves downward, thereby compressing the helical spring 17.
When the switching member 191 is released, the helical spring 192 expands, thereby moving the valve piece 193 to once more block the shaft opening 132. Further downward movement of the outer and inner cylinders, 12 and 11, is stopped, thereby maintaining the chair seat at the desired height.
When the applied downward pressure on the stopper 13 is less than the biasing force of the helical spring 17, and the valve piece 193 is in an open position, the helical spring 17 gradually expands to return the stopper 13 and the outer and inner cylinders, 12 and 11, to the initial unadjusted positions.
The drawback of the above-described conventional height adjusting device 1 is that the axes of the inner cylinder 11, the outer cylinder 12 and the piston shaft 15, must be precisely aligned to ensure proper operation of the device 1. Such precise alignment cannot be achieved conveniently, thereby complicating the manufacturing process of the same.